Fix attribute-specific encryption options

The Active Record encryption guides have two sections that mention
model-specific options: "Model-specific Key Providers" and
"Model-specific Keys". Those options aren't really model-specific, but
attribute-specific.

This commit changes the text from "model" to "attribute", making the
documentation more precise.

Author: neilvcarvalho

guides/source/active_record_encryption.md

@@ -343,19 +343,19 @@ Both methods return `ActiveRecord::Encryption::Key` objects:
 
 A key can include arbitrary tags that will be stored unencrypted with the message. You can use `ActiveRecord::Encryption::Message#headers` to examine those values when decrypting.
 
-### Model-specific Key Providers
+### Attribute-specific Key Providers
 
-You can configure a key provider on a per-class basis with the `:key_provider` option:
+You can configure a key provider on a per-attribute basis with the `:key_provider` option:
 
 ```ruby
 class Article < ApplicationRecord
   encrypts :summary, key_provider: ArticleKeyProvider.new
 end
 ```
 
-### Model-specific Keys
+### Attribute-specific Keys
 
-You can configure a given key on a per-class basis with the `:key` option:
+You can configure a given key on a per-attribute basis with the `:key` option:
 
 ```ruby
 class Article < ApplicationRecord